Interstage transformer for PI.

[jane]

I am looking for an interstage transformer that I can use as a PI. (SE driver -> PP output)

On the primary side I am going to have 2x (small side) 6EM7/6EA7 in common cathode in parallel, each with a plate resistance of ~38k. Max voltage swing is estimated to be 140V p-p.

The two secondary windings will drive two CF's that feeds the output tubes (DC-coupling). This section needs about 120V p-p for full drive.

Wanted frequency response: (-1dB) 15Hz – 30kHz.

Any suggestions for a proper transformer?

[jane]

I have made a sketch over the circuit, and I wonder where I can find an interstage transformer that will fit into this?

Rp = ~38k each driver triode.
f (-1dB) 15Hz-30kHz (15Hz is maybe well optimistic when total Rp is 19k?)
Vac max (primary) = 140v p-p
Vac max (secondary) = 120+120v p-p

[NickC]

For interstages, it is often better to have your rp is the ballpark of 5k and below.

To get proper swing the interstage as to be off around 40k to offer some decent bass.

Also the high rp will limit your hf extension too. Also in your design the IT is used as a step up. This will further limit your freq response.

I can help you with the interstage trans to be wound, but contact me offline on this.

[Eli Duttman]

Perhaps a topology change would permit you to avoid the expense and bother of custom "iron". The Sowter model 8920 is a nice phase splitter, but "standing" DC is not allowed. You could use the 8920 at the amp's I/P, 2X high gain 6EM7 sections wired common cathode/cathode follower in each phase would drive the parallel pairs of low mu/"high" power 6EM7 sections.

[jane]

Very good idea!
That makes it a lot easier to find an interstage transformer, too.
Very good idea, indeed!

If I put the common cathodes between C- and B+, I'll also get the voltage swing that I need.
Great!